THE GEOLOGIC FRAMEWORK OF THE CHANDELEUR ISLANDS, LOUISIANA: INSIGHT INTO THE HISTORY AND FATE OF A FRAGILE ECOSYSTEM

The Chandeleur Islands of Louisiana are preserved natural environments that provide habitat for endangered wildlife species, regulate estuarine salinity levels, support recreational fishing, and protect the mainland from tropical storms and daily wave activity. The islands are deteriorating as subsidence and storm impacts overwhelm the finite sand supply and segment the island chain. In 2005, Hurricanes Katrina and Rita fragmented the islands into splinters of marsh and a mosaic of submerged shoals, reducing land area by 85%.

The U.S. Geological Survey in collaboration with the U.S. Fish and Wildlife Service, Louisiana Department of Natural Resources and the University of New Orleans began in 2006 an assessment of morphology and stratigraphy of the barrier islands to characterize the geologic framework, provide information to model island change, and identify sediment resources for island restoration. Understanding the geologic and oceanographic processes that influence the islands is crucial in determining their stability and fate.

Interpretation of geophysical and sediment-core data shows that the stratigraphy consists primarily of a thick sequence of prodelta and delta front sandy silts, incised by sand and silt-filled distributary channels, associated with the relict St. Bernard Delta complex of the Mississippi River. These deposits are overlain by the modern barrier platform that consists of 85% well-sorted sand. Northward littoral transport has formed a large sandy spit at the northern tip if the islands that contains over 350x10⁶ m³ of sediment.

These data provide insight into the history and fate of the islands. The Chandeleur Islands formed < 3 kya in response to delta abandonment and marine reworking of the deltaic deposits into a transgressive barrier island arc. Most of these deposits have since subsided below depth of wave ravinement, decreasing the sand available to maintain island integrity. Continued storm impacts reduce the available sand in the littoral system and erode the marsh platform that helps to stabilize sand accumulation. As a restoration strategy, sand deposits with similar characteristics to the barrier platform can be found within the buried fluvial channels and northern spit to reintroduce sand lost from the system.